1. Field of the Invention
The present invention relates to an underlying layer of an alignment film for an oxide superconducting conductor, a method of forming the same, and a device for forming the same. In greater detail, it relates to an underlying layer of an alignment film for an oxide superconducting conductor in which two or more kinds of thin films are repeatedly laminated, and a method for forming the same, and a device for forming the same.
2. Description of the Related Art
In order to use an oxide superconductor as a superconducting conductor, it is necessary to form a thin film of an oxide superconductor with good crystal orientation on an elongated, such as tape-shaped, base material.
However, generally a metal tape itself is a polycrystalline body, and since the crystal structure thereof greatly differs from that of an oxide superconductor, it is difficult to directly form a thin film of an oxide superconductor with a good crystal orientation on a metal tape.
Also, since there is difference in the coefficient of thermal expansion and a lattice constant between the base material and the superconductor, in the process of cooling until the superconducting critical temperature, there are also the problems of deformation arising in the superconductor, or the oxide superconductor film exfoliating from the substrate.
Therefore, an intermediate layer (alignment film) is formed on the metal base material, and an oxide superconductor film is formed on this intermediate layer (alignment film). This intermediate layer (alignment film) consists of a material such as MgO, YSZ (yttria-stabilized zirconia), SrTiO3 of which the physical characteristic value such as the coefficient of thermal expansion or lattice constant shows an intermediate value between the base material and the superconductor. The crystal orientation of this intermediate layer (alignment film) has a significant effect on the crystal orientation of the oxide superconductor film that is formed on this intermediate layer.
For that reason, the applicants developed the ion beam assisted deposition (IBAD) method as a film forming method that forms an intermediate layer (alignment film) having a good crystal orientation. This ion beam assist method is a technique of depositing on a base material constituent particles that are ejected from a target by a sputtering method, while simultaneously irradiating from an oblique direction (for example, 45 degrees) argon ions and oxygen ions, etc. which are generated from an ion gun. According to this method, it is possible to form an intermediate layer (alignment film) that has a favorable crystal orientation on a base material.
Furthermore, when forming an oxide superconductive layer (oxide superconductor film) on this intermediate layer (alignment film), it is possible to form an oxide superconductive layer in which the crystal orientation is excellent and the critical current density is high.
Also, consideration has been given to laminating an Al2O3 film and a Y2O3 film in succession on a base material, and forming a intermediate layer (alignment film) thereon by the IBAD method (for example, refer to U.S. Pat. No. 6,921,741 and U.S. Pat. No. 6,933,065). By forming the intermediate layer (alignment film) on the Y2O3 film (bed layer), it is possible to further improve the crystal orientation state. Also, the Al2O3 layer (diffusion prevention layer) is provided in order to prevent the problem of the metal base material constituent element diffusing from the metal base material to the oxide superconductive layer in the high-temperature process during manufacture of the oxide superconducting conductor.
As a method of forming a thin film that can effectively use the film formation area when forming the diffusion prevention layer and the bed layer on a metal tape wire, the applicants proposed a method of forming a thin film by causing a strip-like base material to pass multiple times through deposition regions of vapor-deposition particles, to cause the vapor-deposition particles to be deposited on the strip-like base material in each pass (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2004-263227).
Also, as a method of forming a multi-layer film that has a repeated structure using a plurality of targets, there is a method that controls the operation of the vapor deposition mechanism by providing a shutter or slit in the upper portion of the target (for example, refer to Japanese Patent No. 2558880), and a film formation method that provides a plurality of targets under a cylindrical drum, and rotates the drum with an elongated base material being spirally wound on the cylindrical drum (for example, refer to Japanese Unexamined Patent Application, First Publication No. H05-6586).
Due to such vigorous research and development, technology to make the intermediate layer (alignment film) thinner in the state of having a good crystal orientation has been put to practical use. However, by making the intermediate layer (alignment film) thin, there is the problem of the metal base material constituent element diffusing from the metal base material to the oxide superconductor during the high-temperature process during manufacture of the oxide superconducting conductor.
In order to solve this problem, it is indispensable to insert an underlying layer of the intermediate layer (alignment film) such as a diffusion prevention layer between the metal base material and the intermediate layer (alignment film). Preventing diffusion of a metal base material constituent element can be achieved by increasing the thickness of the film of the diffusion prevention layer. However, since there is the problem of the substrate warping due to an increase in the internal stress of the film when the thickness is increased, it is sought to further increase the diffusion prevention effect of the underlying layer of an alignment film by a method other than increasing the film thickness.
In the related art disclosed in the aforementioned Japanese Unexamined Patent Application, First Publication No. 2004-263227, as a method for causing a strip-like base material to pass multiple times through deposition regions of vapor-deposition particles, a pair of winding member groups, which consist of a plurality of coaxially disposed winding members that wind the strip-like base material, are oppositely arranged, and the base material that is wound on this pair of winding member groups is made to loop around to perform thin-film formation as a plurality of rows within the deposition region of the vapor-deposition particles. According to this related method, it is possible to effectively use the film formation area, but in the case of using two or more kinds of targets, it is necessary to perform two or more film formation processes. According to this related method, in order to form the diffusion prevention layer and the bed layer on the strip-like base material, it is necessary to first install a target for the diffusion prevention layer such as Al2O3 to form the diffusion prevention layer, then replace the target with a target for the bed layer such as Y2O3, so there has been a problem in terms of productivity.
Also, the film formation method disclosed in the aforementioned Japanese Patent No. 2558880, which is a method that forms a multi-layer film having a repeated structure using a plurality of targets, can only be applied to film formation on a plate-like base material, and so cannot be applied as a film-formation method of a multi-layer film to an elongated base material.
On the other hand, in the film formation method disclosed in the aforementioned Japanese Unexamined Patent Application, First Publication No. H05-6586, obtaining a film with a single crystal state having a good crystal orientation is not assumed.